In an internal combustion engine which is installed in a vehicle or the like, there is provided an evaporation fuel control apparatus which prevents the emission of the evaporation fuel which is generated in a fuel tank or the like during stoppage of the engine. In the evaporation fuel control apparatus, a canister is arranged in the way of an air ventilation passage for communicating an intake passage of the engine and the fuel tank, and a purge control valve is arranged in the way of the air ventilation passage between the canister and the intake passage.
In the evaporation fuel control apparatus, the purge control valve is closed by a control unit when the engine is stopped, thereby allowing the evaporation fuel to be temporarily adsorbed and held in the canister. When the engine operates, the purge control valve is duty controlled, thereby purging (removing) the evaporation fuel adsorbed and held in the canister and feeding it to the intake passage.
Such evaporation fuel control apparatus are disclosed in JP-A-62-233466 or JP-A-1-211661.
According to the apparatus disclosed in JP-A-62-33466, a main purge control valve is provided in an air ventilation passage, a sub-purge control valve is provided in a bypass air ventilation passage to bypass the main purge control valve, and in order to prevent that an air fuel ratio becomes overdense or overlean at the start or stop of the purge, a permission or inhibition of the purge is discriminated from a stored air fuel ratio feedback coefficient and a presumed air fuel ratio feedback coefficient, thereby controlling a purge amount.
According to the apparatus disclosed in JP-A-1-211661, in order to prevent the air fuel ratio from becoming overdense in the first purge of the evaporation fuel after the fuel is fed into the fuel tank, a fuel feed amount to the engine is reduced at the time of the first purge of the evaporation fuel.
An evaporation fuel control apparatus also is disclosed in JP-A-2-245461. According to the apparatus disclosed, as the fuel concentration of purge gas (evaporation fuel) is high, the opening operating speed of a purge valve is reduced, thereby preventing the air fuel ratio from transiently becoming rich at the initial stage of the start of the purge.
Further, there is also known an evaporation fuel control apparatus in which in the situation where a cooling water temperature exceeds a set temperature at the start of the cooling of the engine, the evaporation fuel is purged in accordance with a duty map by a purge control valve which is duty controlled.
In the conventional evaporation fuel control apparatus of the engine, however, in the situation where the adsorbing state of the evaporation fuel to the canister is in the extreme over-adsorbing state (as a state in which it exceeds the over-adsorbing state) and the evaporation fuel has been further adsorbed to the canister, the evaporation fuel can be easily purged from the canister.
As mentioned above, in the situation where the canister is in the extreme over-adsorbing state, when the first purge is executed after the start of the engine, a purge amount increases since the evaporation fuel can be easily purged from the canister. In this situation, when the air fuel ratio control is attempted, the air fuel ratio cannot be controlled to a target value.
Therefore, at the time of the purge by the canister in the extreme over-adsorbing state after the start of the engine, there is a large influence on the air fuel ratio control. In particular, the air fuel ratio cannot be controlled to a target value, and the air fuel ratio becomes overdense. As a result, the harmful exhaust component is deteriorated due to the overdense air fuel ratio.